JDK源碼－String的index解讀

                                           String中的index

    String中的索引有indexOf、lastIndexOf，主要對字符、字符串進行索引。此處主要想寫lastIndexOf(String str, int fromIndex)。首先看一段小代碼。此處對應的方法爲lastIndexOf(String str, int fromIndex).

public class StringIndexTest {
    public static void main(String args[]) {
        String source = "DuJingDuJingDu";
        String target = "Jing";
        System.out.println("source string is : " + source + "\n" + "target string is : " + target);
        System.out.println("source.lastIndexOf(target, 1) : " + source.lastIndexOf(target, 1));
        System.out.println("source.lastIndexOf(target, 2) : " + source.lastIndexOf(target, 2));
        System.out.println("source.lastIndexOf(target, 6) : " + source.lastIndexOf(target, 6));
    }
}
/**
 * Result
 * source string is : DuJingDuJingDu
 * target string is : Jing
 * source.lastIndexOf(target, 1) : -1
 * source.lastIndexOf(target, 2) : 2
 * source.lastIndexOf(target, 6) : 2
 */

從代碼可得出以下幾點：

第1：fromIndex,即給出的索引值是倒序的索引值，lastIndexOf是採取例序的方法檢索的；

第2：案例中看出，案例中source.lastIndexOf(target,2),fromIndex是2，此處與target僅僅對應了一個字符'J'，竟然也能索引成功。可知fromIndex只是target首字符的索引起始值。

接下來，從源碼角度分析。lastIndexOf(String str)和lastIndexOf(String str, int fromIndex)全部引用下邊的方法，所以直入正題。

static int lastIndexOf(char[] source, int sourceOffset, int sourceCount,
            char[] target, int targetOffset, int targetCount,
            int fromIndex) 

從源碼一句句來看：

     /**
     * Code shared by String and StringBuffer to do searches. The
     * source is the character array being searched, and the target
     * is the string being searched for.
     *
     * @param   source       the characters being searched.
     * @param   sourceOffset offset of the source string.
     * @param   sourceCount  count of the source string.
     * @param   target       the characters being searched for.
     * @param   targetOffset offset of the target string.
     * @param   targetCount  count of the target string.
     * @param   fromIndex    the index to begin searching from.
     */
    static int lastIndexOf(char[] source, int sourceOffset, int sourceCount,
            char[] target, int targetOffset, int targetCount,
            int fromIndex) {
        /*
         * Check arguments; return immediately where possible. For
         * consistency, don't check for null str.
         */
      //採取倒序的方式檢索，fromIndex爲target首字符的倒序檢索起始位置;
      //因此，若fromIndex對應字符與target首字符相同，但target並不在整個檢索域中，也能檢索成功.
        int rightIndex = sourceCount - targetCount;//計算出最大可能性的閾值
        if (fromIndex < 0) {
            return -1;
        }
        if (fromIndex > rightIndex) {
            fromIndex = rightIndex;//當人爲設置的fromIndex超出閾值時，自動進行調整.
        }
        /* Empty string always matches. */
       //目標字符串爲空，默認檢索成功，返回檢索起始位置fromIndex.
        if (targetCount == 0) {
            return fromIndex;
        }

        int strLastIndex = targetOffset + targetCount - 1;//得出target的最後索引.
        char strLastChar = target[strLastIndex];
        //以最右端對應的索引值，min表示最小索引值，i爲最大索引值.
        int min = sourceOffset + targetCount - 1;
        int i = min + fromIndex;

    startSearchForLastChar:
        while (true) {
            while (i >= min && source[i] != strLastChar) {
                i--;//找到最後一個字符相等的位置.
            }
            if (i < min) {
                return -1;
            }
            //設置起始、終止位置，對兩個字符串進行比較.
            int j = i - 1;
            int start = j - (targetCount - 1);
            int k = strLastIndex - 1;

            while (j > start) {
                if (source[j--] != target[k--]) {
                    i--;//當存在一個字符不相等的情況下，開始進入下一輪循環.
                    continue startSearchForLastChar;
                }
            }
            //當退出循環走到這一步時，表明每個字符都相等，所以返回具體位置.
            return start - sourceOffset + 1;
        }
    }

本來是想畫圖的，但時間較緊，向每一個打算讀的朋友抱歉。歡迎交流，歡迎批評指正！